5 Steps to Replacing Thru Hulls
We’re in the process of replacing all of the below-the waterline thru hulls on the boat. All parts of them, even. The ones we’re taking out are not ideal - they’re straight-thread mushroom (the actual thru hull part) to tapered-thread (or pipe-thread) ball valve. To be clear, we’re the ones who installed all our thru hulls, and we’re also the ones who have done basically no maintenance (NOT advised) in the 30 years since we did so. The new installation will be far safer, using a straight-thread mushroom screwed into the straight-thread part of an adapter flange, which then has a tapered-thread ball valve screwed into the tapered-thread part of that flange. We’re also epoxying fiberglass pucks that provide a flat surface for the flange to grab AND (most critically) a place that the flange can fix securely to the boat; the epoxied puck (with its tapped bolt holes) offers a place to secure the flange to the boat; otherwise you’re either thru-bolting or just crossing your fingers.
Interior view of adapter flange (before bolts) on epoxied puck.
It’s not a particularly difficult project, at least once the old thru hulls are out, though if you have obstruction around where the puck goes it adds some complexity and time. Still, there are a few steps to go through!
Measure the thru hulls to see what replacement parts you need to order. There will be a marking on the old ball valve that indicates size. You might be able to maneuver a phone around to take a photo of the markings, or there’s a sweet little tool that I’ve got in my Amazon cart.
Order your parts. Your choices on material are pretty much bronze or marelon; there are also stainless steel thru hulls available. We use bronze because it’s strong and corrosion-resistant. DO NOT USE BRASS. IF you do not have fiberglass (or some other strong material) pucks already on the hull, which are important to offer a flat surface for the flange to rest on, you’ll either want to make some or buy some. Don Green, over at MeriMarine, has ready-made pucks that are pre-bored for the bolts. Cheap they are not, but the amount of time we’d spend making our own made it a no-brainer to buy his.
Remove the old thru hulls. Sure, removing the ball valve should be relatively simple; it’s taking out the mushrooms that is the tough part. A thru hull removal tool can help, but be advised it’s not a slam dunk. For starters, you’ll need a large crescent wrench or some other tool to actually be able to provide the leverage needed to unscrew the mushroom. And then there’s the “what was the mushroom installed WITH” question. We had to destroy 3 of the 5 thru hulls to get them out, at one point resorting to using a Sawzall.
Thru hull puller, or thru hull removal tool
Clean up the hole, making sure to get any sealant out. You’ll likely need to sand the inside of the hull to make sure the epoxy can grip well; making sure the exterior is clear of any ridges that would keep the thru hull from sitting flush is a good idea as well. A drum sanding attachment for your drill can help clean up the area that actually goes through the hull.
Seat the puck. This involves a few dry-fit sessions, so don’t panic. It’s a heck of a lot easier with 2 people, since one is inside the boat and the other outside, holding the thru hull in place. Essentially, you need to seat the puck correctly, which requires holding it in place between the thru hull and the flange while the epoxy cures. Your thru hull might need to be cut down if it’s too long for your hull thickness. You also may need to bevel part of the puck. Hence the dry fit. We chose to install the puck first and not try to do the installation of all the parts at the same time; I can’t imagine the stress that would come with doing all together. Wrap the threads of the thru hull part with blue tape (to keep epoxy from getting on them), tape the bolt holes (so epoxy doesn’t get into those already-tapped babies). Mix up your epoxy and thicken it. One person will now shove the thru hull in from outside of the hull while the other person blobs on epoxy, seats the puck, and screws on the flange to tight. Clean up any stray epoxy! Let this cure fully, then remove the flange and the thru hull.
You’ll then be able to install the flange and the thru hull, using your preferred sealant. Since we’ve learned that anything we put on a boat will eventually have to be removed, 5200 is not our choice. We’re trying 4200 this time.
Have you replaced your thru hulls? How did it go for you?